System and method for oxygen delignification of pulp made for lignocellulosic material

ABSTRACT

The system is for the oxygen delignification, in at least two reaction stages, of pulp that consists of lignocellulose-containing material having a mean concentration of 8-18% pulp consistency. The system has a first pump followed by a first oxygen mixer that is followed by a first delignification zone. The first delignification zone is followed by a second steam mixer that is followed by a second pump that is followed by a third oxygen mixer and a second delignification zone.

PRIOR APPLICATIONS

This application is a U.S. national phase application based uponInternational Application No. PCT/SE00/01435, filed 5 Jul. 2000; whichclaims priority from Swedish Application No. 9902586-8, filed 6 Jul.1999.

TECHNICAL FIELD

The present invention relates to a system and a process for oxygendelignification.

BACKGROUND AND SUMMARY OF THE INVENTION

A number of different processes for oxygen delignification are known.For example, U.S. Pat. No. 4,259,150 presents a system with multistageoxygen bleaching in which, in each stage, the pulp is first mixed to alower consistency with O₂, water and NaOH, followed by a thickening backto the consistency level which the pulp had prior to the stage inquestion. The aim is to obtain an economic, chlorine-free bleaching withhigh yield. At the same time, the kappa number can be lowered, by meansof repeated stages, from 70 down to 15 or even less than 15.

Swedish Patent C,467.582 presents an improved system for the oxygenbleaching of pulp of medium consistency. By means of controlling thetemperature in an optimized manner, an oxygen bleaching takes place in afirst delignification zone at a low temperature, with this beingfollowed by a second delignification zone at a temperature which is20-40 degrees higher. The aim is to obtain an improved yield and animproved viscosity, while retaining the dwell time, in association withindustrial use.

Other variants of oxygen delignification in two stages have also beenpatented in addition to Swedish Patent No. C,467.582. Swedish Patent No.C,505.147 presents a process in which the pulp should have a high pulpconcentration in the range of 25-40% in the first stage and aconcentration of 8-16% in the second stage, at the same time as thetemperature in the second stage should be higher than, or equal to, thetemperature in the first stage, in line with the temperature differencewhich is recommended in Swedish Patent No. C,467.582. The advantages ofthe solution in accordance with Swedish Patent No. C,505.147 are statedto be the possibilities of admixing more oxygen in the firsthigh-consistency stage without there being any risk of channel formationbut where, at the same time, unused quantities of oxygen can be bled offafter the first stage in order subsequently to be admixed in a secondmixer prior to the second stage.

Swedish Patent No. C,505.141 presents a further process which is anattempt to circumvent Swedish Patent No. C,467.582, since that which itis sought to patent is stated to be that a temperature differencebetween the stages does not exceed 20 degrees, i.e., the lower suitabletemperature difference patented in SE,C,467.582, but that a temperaturedifference should nevertheless be present. In addition to that, it isstated that a) the pressure should be higher in the first stage and b)that the dwell time is short in the first stage, i.e., in the order ofmagnitude of 10-30 minutes, and also c) the dwell time in the secondstage is longer, i.e., in the order of magnitude of 45-180 minutes.

A lecture entitled “Two stage MC-oxygen delignification process andoperating experience” which was given by Shinichiro Kondo from theTechnical Div. Technical Dept. OJI PAPER CO. Ltd. At the 1992Pan-Pacific Pulp & Paper Technology Conference, 99 PAN-PAC PPTC, Sep.8-10, Sheraton Grande Tokyo Bay Hotel & Towers, presents a successfulinstallation which was constructed with two-stage oxygen delignificationin 1986 in a plant in Tomakomai.

In this OJI PAPER plant in Tomakomai, the pulp was fed, with a pressureof 10 bar, to a first oxygen mixer (+team) followed by anafter-treatment in a pre-retention tube (pre-reactor), with a 10 minutedwell time in which the pulp pressure is reduced to a level of about 8-6bar due to pipe losses, etc. After that, the pulp was fed to a secondoxygen mixture followed by an after-treatment in a reactor at a pressureof 5-2 bar and with a dwell time of 60 minutes. It was stated at thispoint that preference would have been given to having a pre-retentiontube which would have given a dwell time of 20 minutes but that it wasnot possible to construct this due to lack of space. The OJI PAPERstated that, by using this installation, they had succeeded in obtainingan increase in kappa reduction at a lower cost in chemicals and with thepulp viscosity being improved.

Most of the prior art has consequently been directed towards a higherpressure in the first reactor at a level of about 6(8)-10 bar. Apressure in the first reactor of up to 20 bar has even been discussed incertain extreme applications. This results in it being necessary tomanufacture the reactor spaces which are required for the firstdelignification zone such that they can cope with these high pressurelevels, with a consequent requirement for substantial material thicknessand/or good material qualities, which in turn result in an expensiveinstallation.

In pulp suspensions in industrial production processes, there are largequantities of readily oxidizable constituents/structures which alreadyreact under modest process conditions. It is therefore advantageous, ina first stage, to add oxygen in quantities which are such that this partof the pulp which is relatively easily oxidized is allowed tooxidize/react first of all. Severe problems arise if an attempt is madeto compensate for this by over-adding oxygen since there is theimmediate danger of canalization problems, as mentioned in SwedishPatent No. C,505.147.

One object of the present invention is to avoid the disadvantages of theprior art and to obtain an oxygen delignification which gives increasedselectivity. The present invention permits an optical practicalapplication of the theories regarding a first rapid phase and a secondslower phase during the oxygen delignification process, with the optimalreaction conditions being different between the phases.

At the high hydroxide ion concentrations and high oxygen partialpressures which are conventionally employed in the first stage, thecarbohydrates are attacked more than is necessary, thereby impairing thequality of the pulp. A lower oxygen partial pressure, and preferably alower temperature as well, in the first stage as compared with thesecond stage decreases the rate of reaction for the breakdown ofcarbohydrates more than it decreases the rate of reaction for thedelignification, leading in turn to an increase in the total selectivityon the pulp after the two stages.

Another object of the present invention is to allow a simpler andcheaper process installation in which at least one pressure vessel, in afirst delignification zone, can be manufactured using thinner materialand/or using a lower material quality which is suitable for a lowerpressure class.

Yet another object is also to make it possible to use steam at moderatepressure especially when there is a need to increase the temperaturesubstantially between the first and second stage and when the pressurein the second stage is considerably higher than that in the first stage.In most cases, the supply of medium-pressure steam and low-pressuresteam is very good in a pulp mill whereas high-pressure steam is inshort supply due to the large number of processes which requirehigh-pressure steam. This also makes it possible to convert existingsingle-vessel delignification systems where, with the previously theprior art for converting to a two-stage design, a restriction has beenimposed by the fact that the prevailing pressure in the plant's steamgrid has not enabled a sufficiently large quantity of steam to beadmixed with the pulp in order to achieve the desired temperature in thesecond delignification stage.

Yet another object is to optimize the mixing process in each positionsuch that only that quantity of chemicals/oxygen is added which isconsumed in the subsequent delignification zone and where the admixtureof chemicals/oxygen does not need to compete with the simultaneousadmixture of steam for the purpose of increasing the temperature to thedesired level. In this way, it is possible to dispense with bleedingsystems for overshooting quantities of oxygen at the same time as it ispossible to reduce the total consumption of oxygen, which in turnreduces the operating costs for the operator of the fibre line and thusshortens the pay-off time. At the same time, it is possible to select asmaller size of dynamic mixer for admixing chemicals, which mixer isdimensioned solely for the volumes of chemicals which are actually beingadmixed.

Yet another object is to increase, in an oxygen delignification systemhaving a certain total volume of the first and second stages, aso-called H factor by operating the first stage for a short time at lowtemperature and operating the second stage for a longer time at a highertemperature. Thus, in connection, for example, with conversions ofexisting single-vessel oxygen delignification stages, a simpleconversion, including a small pre-reactor and a modest increase in thereaction temperature in the existing reactor, can increase the H factorand at the same time improve the selectivity over the oxygen stages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system for oxygen delignification in two stages inaccordance with the invention; and

FIG. 2 diagrammatically shows the kinetics of the oxygen delignificationand the advantages which are gained relative to the prior art withregard to reduction in kappa number and an increased H factor.

DETAILED DESCRIPTION

FIG. 1 shows an installation, according to the present invention, of asystem in an existing plant in which the oxygen delignification processneeded upgrading.

An existing first MC pump 1 (MC=medium consistency, typically a pulpconsistency of 8-18%) is connected to a tipping chute 2 for forwardingto an existing first MC mixer 3. The first mixer 3 is a so-calleddynamic mixer, in which a motor-driven rotor agitates the pulp in atleast one narrow fluidization gap. The dynamic mixer is preferably amixer type which corresponds to that which is shown in U.S. Pat. No.433,920, in which a first cylindrical fluidization zone is formedbetween the rotor and the housing and a second fluidization zone isformed between a radially directed rotor part and housing, which mixeris hereby introduced as a reference. A mechanical agitation is requiredin order to obtain a uniform admixture of the chemical charge inquestion in the whole of the pulp suspension, with the aim of the pulpbeing bleached/treated uniformly throughout the whole of the volume ofthe pulp.

An admixture of chemicals, chiefly oxygen, takes place in the first MCmixer 3, after which the pulp was, in the existing system, fed to anoxygen reactor 6. The combination of a first MC pump 1 followed closelyby an MC mixer 3 can be termed a perfect pair. This is the case sincethe pump primarily pressurizes the pulp flow to a given degree, therebyfacilitating a finely divided supply of the oxygen to the MC mixer whichfollows directly thereafter.

In accordance with the invention, an upgrading of the oxygendelignification process is achieved by introducing a static mixer 8,i.e., a non-rotating or mechanically agitating mixer 8 for increasingthe temperature by means of adding steam. The static mixer 8 ispreferably of a construction which has been shown in SE,C,512.192(=PCT/SE00/00137), where steam is conducted in as thin jets through anumber of holes which are uniformly distributed over the periphery of apulp-conveying pipe, which mixer is hereby introduced as a reference.

The static mixer 8 is arranged directly after the oxygen reactor 6 andfollowed by a second MC pump 4 and a second agitating MC mixer 5, of thesame type as the mixer 3, which acts directly after the MC pump 4. Thesystem is assembled such that the coupling pipe 6 forms a firstdelignification zone between the outlet of the first MC mixer 3 and theinlet of the non-rotating mixer 8, which zone gives rise to a dwell timeR_(T) of 2-20 minutes, preferably 2-10 minutes and even moreadvantageously 3-6 minutes.

The second MC pump 4 is controlled such that the resulting pressure inthe dwell line 6 is preferably in the interval 0-6 bar, preferably 0-4bar. Preferably, the second pump 4 is controlled by means of itsrotational speed being controlled by a control system PC depending onthe pressure which prevails, and is detected, in the firstdelignification zone 6.

The temperature in the whole of the first delignification zone 6 can bekept low, preferably at the level which the system allows without addingsteam, but preferably with the pulp entering the first delignificationzone being at a temperature of about 85° C., ±10° C.

The non-rotating mixer 8 is connected in after the first delignificationzone, as are then the second MC pump 4 followed by the second MC mixer5. This second perfect pair combination is controlled such that theresulting pressure in the oxygen reactor 10, which forms a seconddelignification zone, reaches a level of at least 3 bars over-pressureat the top of the reactor. In conventional applications, the pressure inthe second MC mixer should be at least 4 bar higher than the pressure inthe first MC mixer; alternatively, the increase in pressure in thesecond pump should reach 4 bar. In connection with practicalimplementation in conventional oxygen stages, an initial pressure isobtained within the interval 8-10 bar, corresponding to the pressure atthe inlet to the reactor.

In accordance with the present invention, the temperature of the pulp inthe second delignification zone is increased by supplying steam to thenon-rotating mixer directly after the first delignification zone andbefore the pressure-raising pump 4 comes into play. The steam supply isexpediently controlled using a control system TC, which comprises acontrol valve V on the line 7 for the steam supply and a feeding-backmeasurement of the temperature of the pulp which is leaving the mixer.The temperature is expediently raised to a level of 100° C.±10° C., butpreferably at least 5° C. higher than the temperature in the firstdelignification zone. As a result of the steam being added before thepulp is given the higher pressure which is required for the final phaseof the delignification:

-   -   a higher temperature can be obtained;    -   the pressure of the available steam does not need to be so high;        and    -   the mixers for adding chemicals/admixing oxygen do not need to        be burdened with a supply of steam as well, which will otherwise        reduce their efficiency.

The volume of the second delignification zone, i.e., the second reactor,is expediently designed such that it is at least 10 times greater thanthe volume of the first delignification zone, i.e., a retention time ofat least 20-200 minutes, preferably 20-100 minutes and even moreadvantageously within the range 50-90 minutes.

FIG. 2 diagrammatically shows the kinetics of the oxygen delignificationand the advantages with regard to the principles of kappa numberreduction which are obtained relative to the prior art. Curve P1 showsthe principle of a reaction course during the initial phase of thedelignification. This part of the delignification proceeds relativelyrapidly and is typically essentially complete after a good 20 minutes.

However, after a relatively short time, typically only 5-10 minutes, thefinal phase P2 of the delignification takes over and begins to dominateas far as the resulting delignification of the pulp is concerned. Atypical subdivision of the delignification into two stages in accordancewith the prior art is shown at line A, with stage 1 being to the left ofthe line A and stage 2 being to the right of the line A. It follows fromthis that two different dominating processes, i.e., the initial phase ofthe delignification on the one hand, but also its final phase, actuallytake place in stage 1. It can be concluded from this that it becomesimpossible to optimize the process conditions in stage 1 for both thesedelignification phases.

Instead, a subdivision of the delignification into two stages inaccordance with the invention is shown as a line B, a stage 1 is to theleft of the line B and stage 2 is to the right of the line B. This makesit possible to optimize each stage for the process which dominates inthe stage. The curve H_(A) shows the temperature integral plottedagainst time (the H factor) which is typically obtained whenimplementing a delignification process in two stages in accordance withthe prior art, corresponding to the line A.

As can be seen from the figure, it is possible to use the stagesubdivision in accordance with the invention to obtain an H factor whichis higher than that which is typically obtained in currentinstallations. This can be done without foregoing demands for highselectivity over the oxygen delignification system. The invention alsoopens up ways of upgrading, with a small investment, an existing 1-stageprocess of comparatively low selectivity to a 2-stage system of betterselectivity without having to build a new large reactor or even two suchreactors. According to the present invention, the initial phase of theoxygen delignification is dealt with in the pre-reactor, after which thetemperature in the existing reactor can even be increased, if sorequired, in association with the conversion, and an increased H factorcan in this way be combined with increased selectivity.

The invention can be modified in a number of ways within the context ofthe inventive concept. For example, the first delignification zone canconsist of a pre-retention tube which is vertical but in which thepressure in some part of this pre-retention tube, including its bottom,is at least 4 bar lower than the pressure in the initial part of thesecond delignification zone.

Further delignification zones, or intermediate washing/bleaching orextraction of the pulp, can be introduced between the first and seconddelignification zones according to the invention. For example, a thirdperfect pair combination, i.e., a pump with a mixer following it, can bearranged between the zones. What is essential is that the firstdelignification zone is characterized by a lower pressure, a short dwelltime and a moderate temperature, and that the concluding, finaldelignification zone is characterized by a higher pressure (a pressurewhich is at least 4 bar higher than that of the first zone), a longerdwell time (a dwell time which is at least 10 times longer than that inthe first zone) and an increased temperature (a temperature which ispreferably at least 5 degrees higher than that in the first zone).

Where appropriate, it should be possible to charge a first mixer, or anintermediate mixer in a third perfect pair combination, with oxygen, atleast some part of which is blown off from the reactor 10. The economicbasis for such a recovery of oxygen is poor since the cost of oxygen isrelatively low.

In order to ensure optimal process conditions, one or other, preferablythe second, or both of the MC pumps can be rotation speed-controlled independence on the pressure in the first delignification zone.

The present invention can also be modified by a number of varyingadditions of other chemicals either together with the oxygen orseparately from the addition of oxygen, in a separate adding position,which chemicals are selected and suitable for the specific fibre lineand the pulp quality in question, such as

-   -   alkali/NaOH for adjusting the pH level to that which is suitable        for the pulp quality in question,    -   agents for protecting cellulose, for example MgSO₄ or other        alkaline earth metal ions or compounds thereof;    -   additions of complex agents which are performed prior to adding        oxygen, with subsequent removal of precipitated metals, where        appropriate,    -   chlorine dioxide;    -   hydrogen peroxide or organic or inorganic peracids or salts        thereof;    -   free-radical capturing agents, such as alcohols, ketones,        aldehydes or organic acids; and    -   carbon dioxide or other additives.

Where appropriate, it should also be possible to degas exhaust gases,such as residual gases, in immediate conjunction with the second pump,preferably by means of the pump being provided with internal degassing,preferably a pump termed a degassing pump.

While the present invention has been described in accordance withpreferred compositions and embodiments, it is to be understood thatcertain substitutions and alterations may be made thereto withoutdeparting from the spirit and scope of the following claims.

1-31. (canceled)
 32. A method for the oxygen delignification of pulp,comprising: providing a lignocellulose-containing pulp having a meanconcentration is 8-18%; in a first pump, pumping the pulp to a firstmixer; in the first mixer, receiving the pulp and admixing bleachingchemicals to the pulp; in a first delignification zone, delignifying thepulp received from the first mixer, maintaining a first temperature of75-95 C and a first pressure of 0-6 bar for 2-10 minutes; in a secondmixer, receiving the pulp from the first delignification zone andadmixing steam to the pulp to increase the first temperature to a secondtemperature of 90-110 C; in a second pump, receiving the pulp from thesecond mixer, pumping the pulp to a third mixer and increasing the firstpressure to a second pressure of 8-10 bar; in the third mixer, receivingthe pulp from the second pulp and admixing bleaching chemicals to thepulp; and in a second delignification zone, delignifying the pulpreceived from the third mixer for 20-100 minutes.
 33. The methodaccording to claim 32 wherein the method further comprises mechanicallyagitate the pulp in the first and third mixers.
 34. The method accordingto claim 32 wherein the method further comprises adjusting a steamsupply depending upon a temperature of the pulp downstream of the secondmixer.
 35. The method according to claim 34 wherein the method furthercomprises providing the second mixer with a plurality of inlet holes ina pipe wall and sending steam through the inlet holes.
 36. The methodaccording to claim 35 wherein the method further comprises providingsteam at a pressure of 8-14 bar.
 37. The method according to claim 32wherein the method further comprises controlling a rotational speed ofthe second pump and adjusting the rotational speed depending upon apressure in the firs delignification zone.
 38. The method according toclaim 32 wherein the method further comprises adding oxygen to the firstand third mixers.